The cause of flood basalts is a big question in science. Two important facts: first, the mantle of the Earth is solid, but it flows very slowly, and second, most magma on Earth is produced by bringing hot solid rock closer to the Earth’s surface and thus releasing pressure from it (not by just heating it up, as we do to melt things here at the surface). A leading idea is that plumes of hot solid rock occasionally rise up through the Earth’s mantle from near its base. The plume begins to melt by pressure release as it rises toward the bottom of the tectonic plate. The resulting magma percolates upward through the plate and eventually erupts.